Hemodialysis has developed into a life saving procedure for several hundred thousand patients worldwide. The economic costs have risen dramatically because it is a chronic treatment. To secure treatments for the growing number of patients in the industrial nations and to make treatment possible in emergent countries it is necessary to improve the quality of the method and reduce the costs. Optmization of the method influences costs significantly because a well-treated patient is less morbid and needs less care.
In the USA, the NCDS (National Cooperative Dialysis Study) studied the morbidity of a large patient collective as a function of the dialysis dose. Gotch and Sargent (Kidney International 28, 526-534, 1985) found a simple interpretation for the results: The morbidity decreases from a high value to a low constant one when Kt/V increases from 0.8 to  greater than =1. K is the effective clearance for urea, t is the treatment time and V is the total body water.
The hypothesis that morbidity and mortality are related to Kt/V for urea has been corrobated for the uniform treatment market in the USA but not the interpretation by Sargent and Gotch. New data indicate that the mortality decreases further up to a Kt/V of 1.5. No reliable data are available for greater values because of lack of sufficient patient numbers (See: Parker Thomas F. Short-Time Dialysis Should Be Used Only With Great Caution. Seminars in Dialysis 1993;6:164-167; Hakim R M, Breyer J, Ismail N, Schulman G. Effects of dose of dialysis on morbidity and mortality. Am J Kidney Dis 1994; 23: 670-80 and others).
This understanding has resulted in the development of guidelines in the USA (DOQI guidelines) demanding a minimum dose of Kt/V=1.2 generally and 1.4 for diabetics respectively. These guidelines are now regarded as relevant by the supervising authorities and compliance with these minimum requirements must be demonstrated with the help of appropriate methods.
One possibility is measurement of the effective clearance, the treatment time and the total body water. Measurement of treatment time is trivial, total body water can be measured with known methods, e.g., bioimpedance or with urea kinetic modeling. The effective clearance is difficult to measure with conventional means but in the German patent application DE3938662 the inventor has described a method for in-vitro measurement of the effective electrolyte dialysance that is equal to the effective clearance for urea within the errors of measurement. In-vitro and in-vivo experiments have shown that this method can be performed in practice. Industry advertises this method in conferences, e.g., the EDTA congress in Geneva 1997 and the ASN congress in San Antonio 1997.
The assumption made above that electrolyte dialysance is almost equal to urea dialysance is only correct if the electrolyte is a mixture usually called xe2x80x9cacid concentratexe2x80x9d consisting essentially of chlorides. Altering, e.g., the concentration of the total dialysate or the bicarbonate component leads to degradation of agreement.
The method described in DE3938662 calculates the dialysance from the electrolyte transfer measured at two (or more) electrolyte input concentrations. Input and output electrolyte concentrations have to be kept constant over a time period of approximately one to five minutes. Variations result in an error because of the time constant of the measurement. The time constant originates from the filling volume of the dialyzer and results in a delayed and slow settling of the output concentration after a step function on the input. Such a curve is shown in a paper of the inventor: Polaschegg H D, Levin N W. Hemodialysis Machines and Monitors. Jacobs C, Kjellstrand C M, Koch K M, Winchester J F, editors. Replacement of renal function by dialysis, 4th ed. Kluwer academic publishers, 1996:333-79. This book contains all further information regarding the state of the art that is required for the understanding of this description. The variation of the electrolyte concentration is normally done automatically by variation of the mixing ratio between concentrate and water in the dialysis machine. Because the adjustment of a new electrolyte concentration is also subject to a time constant the total measurement takes several minutes and is coupled with a substantial electrolyte transfer that can be compensated subsequently by a controlled addition or subtraction. In general, the method can be used only in newly designed machines. Retrofitting existing machines is laborious and is not offered (by industry). Because of the relatively long measuring time only effective clearance but not dialyzer clearance can be measured with this method. The two values differ by the influence of the recirculation in the blood access and in the circulatory system respectively, a parameter of interest in itself. In any event, the method is only applicable for single patient machines but not for central dialysate supply systems if the modification of the dialysate concentration is done by the mixing system of the dialysis machine. In summary the following disadvantages and limitations apply for the method described (previously) by the inventor that is already in use:
The measurement takes a relatively long time and is related to a non-negligible electrolyte transfer. The method is not applicable to substances not contained in the dialysate (e.g., creatinine, phosphate). The method cannot differentiate between dialyzer clearance and effective clearance.
The goal of this invention is to minimize these disadvantages and in addition to describe a method that can be adapted easily to existing machines without adaptation of the electronic control and that can be used for other substances as well. The method according to the invention is characterized by adding a pre determined amount of a substance to the dialysate circuit upstream of the dialyzer and by measuring the amount of said substance leaving the dialyzer downstream in the dialysate and calculating the dialysance or clearance respectively for the substance added.
The addition of the substance can be done in different ways. Also, a dilution instead of an addition is possible. It is possible to inject a liquid concentrate of a single substance (e.g., NaCl or creatinine) upstream of the dialyzer. Alternatively water only can be injected. Injection can be done manually with a syringe or semiautomatic with a spring operated syringe or automatic with the help of a pump. Also, the concentrate pump of a dialysis machine can be operated at a higher rate for a short period to create an electrolyte bolus. Alternatively to addition of a predetermined amount of a substance a non-predetermined amount can be added that is measured by a sensor arranged upstream of the dialyzer. It is further possible to calibrate a not precisely known but repeatable injectable amount by a first bolus injection with the second injection used for the measurement. The calibration injection is done downstream of the dialyzer but upstream of the sensor or, alternatively, the dialysate circuit is switched into bypass and the calibration injection is done at the same point as the injection for the measurement.
Alternatively to the injection of a liquid substance the addition of an amount of a substance is possible by conveying the dialysate through a powder cartridge or powder bag for a short period.